WO2016204531A1

WO2016204531A1 - Method and device for performing adaptive filtering according to block boundary

Info

Publication number: WO2016204531A1
Application number: PCT/KR2016/006405
Authority: WO
Inventors: 남정학; 김철근
Original assignee: 엘지전자(주)
Priority date: 2015-06-16
Filing date: 2016-06-16
Publication date: 2016-12-22
Also published as: US20180199034A1; US10448015B2

Abstract

The present invention relates to a method for performing adaptive filtering according to a block boundary, comprising the steps of: determining whether a current pixel is within a block boundary; determining the shape of a filter depending on the location of the current pixel, when the current pixel is within the block boundary; calculating a filter coefficient based on the shape of the filter; and performing adaptive loop filtering using the filter coefficient.

Description

Method and apparatus for adaptive filtering based on block boundary

The present invention relates to a method and apparatus for encoding / decoding a video signal, and more particularly, to a technique for adaptively performing filtering according to a block boundary.

Compression coding refers to a series of signal processing techniques for transmitting digitized information through a communication line or for storing in a form suitable for a storage medium. Media such as an image, an image, an audio, and the like may be a target of compression encoding. In particular, a technique of performing compression encoding on an image is called video image compression.

Next-generation video content will be characterized by high spatial resolution, high frame rate and high dimensionality of scene representation. Processing such content would result in a tremendous increase in terms of memory storage, memory access rate, and processing power.

Accordingly, there is a need to design coding tools for more efficiently processing next generation video content.

In particular, it is necessary to increase the compression efficiency by reducing the error of loop filtering at the block boundary for the image that has been decoded.

The present invention proposes a block boundary aware asymmetric adaptive loop filter method that minimizes the error of the adaptive loop filter at the block boundary having different characteristics.

The present invention proposes a block boundary determination method for boundary aware asymmetric adaptive loop filter.

The present invention proposes a filter shape determination method for a boundary-cognitive asymmetric adaptive loop filter.

The present invention proposes a filter coefficient derivation method for boundary-aware asymmetric adaptive loop filter.

The present invention proposes a method of applying an adaptive loop filter for a boundary aware asymmetric adaptive loop filter.

The present invention provides a method for adaptively filtering according to a block boundary, comprising: determining whether a current pixel is included in a block boundary; If the current pixel is included in a block boundary, determining a filter shape according to the position of the current pixel; Calculating filter coefficients based on the filter shape; And performing adaptive loop filtering using the filter coefficients.

The present invention may further include checking whether the at least one other condition related to the block characteristic is satisfied, and when the at least one other condition is satisfied, the adaptive loop filtering is performed. do.

Further, in the present invention, at least one other condition related to the block characteristic is characterized by indicating whether the inter-block prediction mode is different.

Further, in the present invention, at least one other condition related to the block characteristic is characterized by indicating whether the motion vector difference between blocks is smaller than a predetermined threshold value.

Further, in the present invention, at least one other condition related to the block characteristic is characterized by indicating whether or not a residual signal exists in an adjacent block.

Further, in the present invention, at least one other condition related to the block characteristic is characterized by indicating whether adaptive loop filtering is not applied.

Further, in the present invention, the filter shape is determined by reference pixels that are limitedly used, and the reference pixels are changed according to the position of the current pixel.

Further, in the present invention, the limitedly used reference pixels are determined in units of columns adjacent to the block boundary.

Further, in the present invention, the limitedly used reference pixels may be determined in proportion to the position where the current pixel is separated from a block boundary.

Further, in the present invention, the filter shape may be derived according to a method defined in the decoder or determined by receiving information on the filter shape from the encoder.

Further, in the present invention, the filter coefficient is characterized in that it is calculated using a scaling factor corresponding to the filter shape.

Also, the present invention provides an apparatus for adaptively filtering according to a block boundary, the apparatus comprising: a block boundary determination unit determining whether a current pixel is included in a block boundary; A filter shape determiner configured to determine a filter shape according to a position of the current pixel when the current pixel is included in a block boundary; A filter coefficient calculator for calculating filter coefficients based on the filter shape; And a filtering performing unit performing adaptive loop filtering using the filter coefficients.

Further, in the present invention, the block boundary determination unit determines whether at least one other condition related to a block characteristic is satisfied, and when the at least one other condition is satisfied, the adaptive loop filtering is performed. do.

Further, in the present invention, at least one other condition related to the block characteristic is whether the inter-block prediction mode is different, whether the inter-block motion vector difference is smaller than a predetermined threshold value, and whether there is a residual signal in an adjacent block. Whether or not, or whether adaptive loop filtering is not applied.

The present invention can solve the problem of performance degradation at the block boundary when using the adaptive loop filter, and can improve the quality of the reconstructed image.

In addition, the present invention can increase the compression efficiency by reducing the error of loop filtering at the block boundary by applying a block boundary recognition asymmetric adaptive loop filter to the image of the image decoding is completed.

1 is a schematic block diagram of an encoder in which encoding of a video signal is performed as an embodiment to which the present invention is applied.

2 is a schematic block diagram of a decoder in which decoding of a video signal is performed as an embodiment to which the present invention is applied.

3 is a schematic internal block diagram of an in-loop filtering unit as an embodiment to which the present invention is applied.

4 is a schematic internal block diagram of an adaptive loop filtering unit according to an embodiment to which the present invention is applied.

5 is a diagram for describing a division structure of a coding unit according to an embodiment to which the present invention is applied.

FIG. 6 is an embodiment to which the present invention is applied and is a view for explaining an example of applying an adaptive loop filter (ALF) in block units.

FIG. 7 is a diagram for describing an ALF applied at a block boundary as an embodiment to which the present invention is applied.

8 is a flowchart illustrating a block boundary aware asymmetric adaptive loop filtering according to an embodiment to which the present invention is applied.

9 is an embodiment to which the present invention is applied and shows a flowchart for determining a block boundary.

10 to 12 illustrate filter shapes for applying block boundary aware asymmetric adaptive loop filtering as embodiments to which the present invention is applied.

Hereinafter, the configuration and operation of the embodiments of the present invention with reference to the accompanying drawings, the configuration and operation of the present invention described by the drawings will be described as one embodiment, whereby the technical spirit of the present invention And its core composition and operation are not limited.

In addition, the terminology used in the present invention was selected as a general term widely used as possible now, in a specific case will be described using terms arbitrarily selected by the applicant. In such a case, since the meaning is clearly described in the detailed description of the part, it should not be interpreted simply by the name of the term used in the description of the present invention, and it should be understood that the meaning of the term should be understood and interpreted. .

In addition, terms used in the present invention may be replaced for more appropriate interpretation when there are general terms selected to describe the invention or other terms having similar meanings. For example, signals, data, samples, pictures, frames, blocks, etc. may be appropriately replaced and interpreted in each coding process. In addition, partitioning, decomposition, splitting, and division may be appropriately replaced and interpreted in each coding process.

Referring to FIG. 1, the encoder 100 may include an image splitter 110, a transformer 120, a quantizer 130, an inverse quantizer 140, an inverse transformer 150, and an in-loop filter 160. And a decoded picture buffer (DPB) 170, a predictor 180, and an entropy encoder 190. The predictor 180 may include an inter predictor 181 and an intra predictor 185.

The image divider 110 may divide an input image (or a picture or a frame) input to the encoder 100 into one or more processing units. For example, the processing unit may be a Coding Tree Unit (CTU), a Coding Unit (CU), a Prediction Unit (PU), or a Transform Unit (TU).

However, the terms are only used for the convenience of description of the present invention, the present invention is not limited to the definition of the terms. In addition, in the present specification, for convenience of description, the term coding unit or target unit is used as a unit used in encoding or decoding a video signal, but the present invention is not limited thereto and may be appropriately interpreted according to the present invention. will be.

The encoder 100 may generate a residual signal by subtracting a prediction signal output from the inter predictor 181 or the intra predictor 185 from the input image signal, and generate the residual signal. Is transmitted to the converter 120.

The transformer 120 may generate a transform coefficient by applying a transform technique to the residual signal. The conversion process may be applied to pixel blocks having the same size as the square, or may be applied to blocks of variable size rather than square.

The quantization unit 130 may quantize the transform coefficients and transmit the quantized coefficients to the entropy encoding unit 190, and the entropy encoding unit 190 may entropy code the quantized signal and output the bitstream.

The quantized signal output from the quantization unit 130 may be used to generate a prediction signal. For example, the quantized signal may restore the residual signal by applying inverse quantization and inverse transformation through the inverse quantization unit 140 and the inverse transform unit 150 in the loop. A reconstructed signal may be generated by adding the reconstructed residual signal to a prediction signal output from the inter predictor 181 or the intra predictor 185.

Meanwhile, in the compression process as described above, adjacent blocks are quantized by different quantization parameters, thereby causing deterioration of the block boundary. This phenomenon is called blocking artifacts, which is one of the important factors in evaluating image quality. In order to reduce such deterioration, a filtering process may be performed. Through this filtering process, the image quality can be improved by removing the blocking degradation and reducing the error of the current picture.

The in-loop filtering unit 160 applies filtering to the reconstruction signal and outputs it to the reproduction apparatus or transmits it to the decoded picture buffer 170. That is, the in-loop filtering unit 160 may perform a filtering process to minimize the difference between the original image and the reconstructed image.

The filtered signal transmitted to the decoded picture buffer 170 may be transmitted to a prediction filtering unit (not shown) and filtered again to improve prediction performance. The prediction filtering unit may perform a filtering process to minimize the difference between the original image and the reference image. For example, the prediction filtering unit may perform filtering using a Wiener filter.

The signal stored in the decoded picture buffer 170 may be transmitted to the predictor 180 and used to generate a prediction signal. For example, the filtered signal may be used as a reference picture in the inter prediction unit 181. As such, the coding efficiency can be improved by using the filtered picture as a reference picture in the inter prediction mode. The prediction filtering unit may be shown as a separate unit from the prediction unit 180, or may be located in the prediction unit 180 or may be configured together with another unit.

The decoded picture buffer 170 may store the in-loop filtered picture or the predictive filtered picture for use as a reference picture in the inter prediction unit 181.

The inter prediction unit 181 performs temporal prediction and / or spatial prediction to remove temporal redundancy and / or spatial redundancy with reference to the filtered picture through the reconstructed picture or the predictive filtering unit 175. Here, since the reference picture used to perform the prediction is a transformed signal that has been quantized and dequantized in units of blocks at the time of encoding / decoding in the previous time, blocking artifacts or ringing artifacts may exist. have.

Accordingly, the inter prediction unit 181 may interpolate the signals between pixels in sub-pixel units by applying a lowpass filter to solve performance degradation due to discontinuity or quantization of such signals. Herein, the subpixel refers to a virtual pixel generated by applying an interpolation filter, and the integer pixel refers to an actual pixel existing in the reconstructed picture. As the interpolation method, linear interpolation, bi-linear interpolation, wiener filter, or the like may be applied.

The interpolation filter may be applied to a reconstructed picture to improve the precision of prediction. For example, the inter prediction unit 181 generates an interpolation pixel by applying an interpolation filter to integer pixels, and uses an interpolated block composed of interpolated pixels as a prediction block. You can make predictions.

The intra predictor 185 may predict the current block by referring to samples around the block to which current encoding is to be performed. The intra prediction unit 185 may perform the following process to perform intra prediction. First, reference samples necessary for generating a prediction signal may be prepared. The prediction signal may be generated using the prepared reference sample. Then, the prediction mode is encoded. In this case, the reference sample may be prepared through reference sample padding and / or reference sample filtering. Since the reference sample has been predicted and reconstructed, there may be a quantization error. Accordingly, the reference sample filtering process may be performed for each prediction mode used for intra prediction to reduce such an error.

A prediction signal generated through the inter predictor 181 or the intra predictor 185 may be used to generate a reconstruction signal or to generate a residual signal.

2, the decoder 200 includes an entropy decoding unit 210, an inverse quantization unit 220, an inverse transform unit 230, an in-loop filtering unit 240, and a decoded picture buffer unit (DPB). 250, the prediction unit 260 may be configured. The predictor 260 may include an inter predictor 261 and an intra predictor 265.

The reconstructed video signal output through the decoder 200 may be reproduced through the reproducing apparatus.

The decoder 200 may receive a signal output from the encoder 100 of FIG. 1, and the received signal may be entropy decoded through the entropy decoding unit 210.

The inverse quantization unit 220 obtains a transform coefficient from the entropy decoded signal using the quantization step size information.

The inverse transform unit 230 inversely transforms the transform coefficient to obtain a residual signal.

A reconstructed signal is generated by adding the obtained residual signal to a prediction signal output from the inter predictor 260 or the intra predictor 265.

The in-loop filtering unit 240 may perform filtering using the filter parameter, and the filtered reconstruction signal may be output to the playback device or stored in the decoded picture buffer 250. That is, the in-loop filtering unit 240 may perform a filtering process to minimize the difference between the original image and the reconstructed image. In this case, the filter parameter may be transmitted from an encoder or derived from other coding information.

The filtered signal transmitted to the decoded picture buffer 250 may be transmitted to a prediction filtering unit (not shown) and filtered again to improve prediction performance. The prediction filtering unit may perform a filtering process to minimize the difference between the original image and the reference image. For example, the prediction filtering unit may perform filtering using a Wiener filter.

The filtered signal may be transmitted to the predictor 260 to be used to generate a prediction signal. For example, the signal filtered by the prediction filtering unit 255 may be used as a reference picture in the inter prediction unit 261. As such, the coding efficiency can be improved by using the filtered picture as a reference picture in the inter prediction mode.

The decoded picture buffer 250 may store the in-loop filtered picture or the predictive filtered picture for use as a reference picture in the inter prediction unit 261.

In the present specification, the embodiments described in the in-loop filtering unit 160, the inter prediction unit 181, and the intra prediction unit 185 of the encoder 100 are each an in-loop filtering unit 240 and an inter prediction unit of the decoder. The same may be applied to the 261 and the intra predictor 265.

The in-loop filtering unit may include at least one of a deblocking filtering unit 310, an adaptive offset filtering unit 320, and an adaptive loop filtering unit 330.

The in-loop filtering unit may apply filtering to the reconstructed picture and output the filtered picture to the reproduction device or store the result in a buffer to use as a reference picture in the inter prediction mode.

The deblocking filtering unit 310 performs a function of improving distortion occurring at the boundary of the reconstructed picture. For example, blocking degradation occurring at the boundary of the prediction unit or the transform unit may be improved.

First, the deblocking filtering unit 310 may determine whether a reconstructed pixel value is discontinuous at a block boundary, and perform deblocking filtering at a corresponding edge boundary when blocking degradation occurs. For example, it may be determined whether a block boundary is an 8x8 block boundary and a boundary of a prediction unit or a transform unit, and a boundary strength BS value may be calculated based on the block boundary. It may be determined whether to perform filtering based on the boundary strength (BS) value, and the filtering parameter may be used together.

The adaptive offset filtering unit 320 may perform a function of minimizing an error between the reconstructed image and the original image by adding an offset to the reconstructed pixel. Here, the reconstructed image may mean a deblocking filtered image. In the case of the encoder, an offset parameter for correcting an error between the reconstructed image and the original image may be calculated and transmitted to the decoder. In the case of the decoder, the transmitted offset parameter may be entropy decoded and then filtered based on the pixel unit.

The adaptive loop filtering unit 330 may perform filtering by calculating an optimal coefficient that minimizes an error between the original image and the reconstructed image. In the case of the encoder, it is possible to derive filter coefficients that minimize the error between the original image and the reconstructed image, and adaptively transmit information and filter coefficients about whether adaptive loop filtering is applied to each decoder to the decoder. In the case of the decoder, filtering may be performed based on information on whether the transmitted adaptive loop filtering is applied and filter coefficients.

In one embodiment of the present invention, the adaptive loop filtering unit 330 provides a block boundary aware asymmetric adaptive loop filter method for minimizing the error of the adaptive loop filter at the block boundary having different characteristics.

In one embodiment of the present invention, the adaptive loop filtering unit 330 provides a method for determining a block boundary for block boundary aware asymmetric adaptive loop filter.

In one embodiment of the present invention, a method for determining a filter shape for a block boundary aware asymmetric adaptive loop filter is provided.

In one embodiment of the present invention, a method for deriving filter coefficients for block boundary aware asymmetric adaptive loop filter is provided.

The adaptive loop filtering unit 330 may include at least one of a block boundary determiner 331, a filter shape determiner 332, a filter coefficient calculator 333, and a filter performer 334.

The block boundary determination unit 331 determines whether the target pixel for applying the current adaptive loop filter belongs to the block boundary. For example, if the location of the target pixel to be currently filtered is determined as the block boundary, filtering embodiments to which the present invention is applied may be applied. On the other hand, if it is not a block boundary, the existing loop filtering method may be applied.

The filter shape determiner 332 may determine the filter shape when the position of the target pixel to be currently filtered is determined as the block boundary. For example, the filter shape may be determined according to the position of the target pixel. In this case, the filter shape may vary depending on the position of the target pixel. In addition, the filter shape may be derived according to a method defined in the decoder, or may be determined by receiving information on the filter shape from an encoder.

The filter coefficient calculator 333 may calculate the filter coefficients based on the shape determined by the filter shape determiner 332.

The filtering performing unit 334 may perform filtering based on at least one of the filter shape or the filter coefficients.

The encoder may split one image (or picture) in units of a rectangular Coding Tree Unit (CTU). In addition, one CTU is sequentially encoded according to a raster scan order.

For example, the size of the CTU may be set to any one of 64x64, 32x32, and 16x16, but the present invention is not limited thereto. The encoder may select and use the size of the CTU according to the resolution of the input video or the characteristics of the input video. The CTU may include a coding tree block (CTB) for a luma component and a coding tree block (CTB) for two chroma components corresponding thereto.

One CTU may be decomposed into a quadtree (QT) structure. For example, one CTU may be divided into four units having a square shape and each side is reduced by half in length. The decomposition of this QT structure can be done recursively.

Referring to FIG. 5, a root node of a QT may be associated with a CTU. The QT may be split until it reaches a leaf node, where the leaf node may be referred to as a coding unit (CU).

A CU may mean a basic unit of coding in which an input image is processed, for example, intra / inter prediction is performed. The CU may include a coding block (CB) for a luma component and a CB for two chroma components corresponding thereto. For example, the size of the CU may be determined as any one of 64x64, 32x32, 16x16, and 8x8. However, the present invention is not limited thereto, and in the case of a high resolution image, the size of the CU may be larger or more diverse.

Referring to FIG. 5, the CTU corresponds to a root node and has a smallest depth (ie, level 0) value. The CTU may not be divided according to the characteristics of the input image. In this case, the CTU corresponds to a CU.

The CTU may be decomposed in QT form, and as a result, lower nodes having a depth of level 1 may be generated. And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a depth of level 1 corresponds to a CU. For example, in FIG. 5 (b), CU (a), CU (b), and CU (j) corresponding to nodes a, b, and j are divided once in the CTU and have a depth of level 1. FIG.

At least one of the nodes having a depth of level 1 may be split into QT again. And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a level 2 depth corresponds to a CU. For example, in FIG. 5 (b), CU (c), CU (h) and CU (i) corresponding to nodes c, h and i are divided twice in the CTU and have a depth of level 2. FIG.

In addition, at least one of the nodes having a depth of 2 may be divided into QTs. And, a node that is no longer partitioned (ie, a leaf node) in a lower node having a depth of level 3 corresponds to a CU. For example, in FIG. 5 (b), CU (d), CU (e), CU (f), and CU (g) corresponding to nodes d, e, f, and g are divided three times in the CTU, and level 3 Has a depth of.

In the encoder, the maximum size or the minimum size of the CU may be determined according to characteristics (eg, resolution) of the video image or in consideration of encoding efficiency. Information about this or information capable of deriving the information may be included in the bitstream. A CU having a maximum size may be referred to as a largest coding unit (LCU), and a CU having a minimum size may be referred to as a smallest coding unit (SCU).

In addition, a CU having a tree structure may be hierarchically divided with predetermined maximum depth information (or maximum level information). Each partitioned CU may have depth information. Since the depth information indicates the number and / or degree of division of the CU, the depth information may include information about the size of the CU.

Since the LCU is divided into QT forms, the size of the SCU can be obtained by using the size and maximum depth information of the LCU. Or conversely, using the size of the SCU and the maximum depth information of the tree, the size of the LCU can be obtained.

For one CU, information indicating whether the corresponding CU is split may be delivered to the decoder. For example, the information may be defined as a split flag and may be represented by a syntax element "split_cu_flag". The division flag may be included in all CUs except the SCU. For example, if the split flag value is '1', the corresponding CU is divided into four CUs again. If the split flag value is '0', the CU is not divided any more and the coding process for the CU is not divided. Can be performed.

In the embodiment of FIG. 5, the division process of the CU has been described as an example, but the QT structure described above may also be applied to the division process of a transform unit (TU) which is a basic unit for performing transformation.

The TU may be hierarchically divided into a QT structure from a CU to be coded. For example, a CU may correspond to a root node of a tree for a transform unit (TU).

Since the TU is divided into QT structures, the TU divided from the CU may be divided into smaller lower TUs. For example, the size of the TU may be determined by any one of 32x32, 16x16, 8x8, and 4x4. However, the present invention is not limited thereto, and in the case of a high resolution image, the size of the TU may be larger or more diverse.

For one TU, information indicating whether the corresponding TU is divided may be delivered to the decoder. For example, the information may be defined as a split transform flag and may be represented by a syntax element "split_transform_flag".

The division conversion flag may be included in all TUs except the TU of the minimum size. For example, if the value of the division conversion flag is '1', the corresponding TU is divided into four TUs again. If the value of the division conversion flag is '0', the corresponding TU is no longer divided.

As described above, a CU is a basic unit of coding in which intra prediction or inter prediction is performed. In order to code an input image more effectively, a CU may be divided into prediction units (PUs).

The PU is a basic unit for generating a prediction block, and may generate different prediction blocks in PU units within one CU. The PU may be divided differently according to whether an intra prediction mode or an inter prediction mode is used as a coding mode of a CU to which the PU belongs.

The adaptive loop filter to which the present invention is applied is a technique for filtering the decoded reconstructed image by the filter coefficient received from the encoder. The filter coefficients transmitted from the encoder obtain filter coefficients that minimize errors of the reconstructed picture and the original picture in picture units. The filter coefficient may have an integer form or a real form, and may send N filters having an Mx1 or MxM shape.

In addition, according to an embodiment of the present invention, the adaptive loop filter may determine whether to apply the adaptive loop filter in a large coding unit (LCU) or a coding unit (CU).

Referring to FIG. 6, an example of applying ALF on or ALF off in a block unit is shown. For example, blocks A, C, G, H, I, J, L indicate that an adaptive loop filter is applied and blocks B, D, E, F, K indicate that an adaptive loop filter is not applied. .

In this case, blocks to which the adaptive loop filter is applied and blocks not to be applied, such as blocks A and B, blocks F and G, may have different characteristics. When a block to which the adaptive loop filter is applied performs filtering, filtering performance may be degraded by using pixels of adjacent blocks to which the adaptive loop filter is not applied. For example, when the block G performs filtering using the pixels of the block F, the filtering performance may be degraded.

Accordingly, the present invention provides a method for minimizing the error of adaptive loop filtering at the block boundary.

One embodiment of the present invention provides a method for performing adaptive loop filtering when a block to which an adaptive loop filter is applied and a block to which no adaptive loop filter is applied are adjacent to each other.

Referring to FIG. 7, an example in which an ALF having a size of 7 × 7 is applied at a block boundary is shown. The F block is an ALF off block and the G block is an ALF on block. The current pixel (dark hatched) at the boundary of the G block is filtered using the pixels around it, including the reference pixel (light hatched) of the F block.

However, as described above, since blocks F and G may have different characteristics, when the adaptive loop filtering is performed on the block F, using the reference pixels of the adjacent block F may reduce the filtering performance.

Accordingly, the present invention provides a method for performing asymmetric adaptive loop filtering based on block boundaries.

One embodiment of the present invention provides a block boundary aware asymmetric adaptive loop filter process. The present invention may largely include a block boundary determining step, a filter shape determining step, a filter coefficient calculating step, and a filtering performing step. This embodiment may be performed by an adaptive loop filtering unit in an encoder or a decoder.

First, the adaptive loop filtering unit may check whether the target pixel to be filtered is included in the block boundary (S810). Here, the block boundary may correspond to any one of a picture boundary, an LCU boundary, a CU boundary, a TU boundary, a PU boundary, and the like.

In another embodiment, the adaptive loop filtering unit may determine whether to perform block boundary recognition asymmetric adaptive loop filtering to which the present invention is applied by checking other conditions. Here, block boundary aware asymmetric adaptive loop filtering means applying an adaptive loop filter asymmetrically at the block boundary.

For example, even if the target pixel to be filtered is included in the block boundary, the adaptive loop filtering unit may perform block boundary aware asymmetric adaptive loop filtering to which the present invention is applied when other conditions are satisfied.

Here, the other condition may be a characteristic of a block or a unit, and for example, a prediction mode, a motion vector difference between blocks, a presence of a residual signal, and whether an ALF is applied.

In another embodiment, the adaptive loop filtering unit may perform block boundary aware asymmetric adaptive loop filtering based on flag information indicating whether to perform block boundary aware asymmetric adaptive loop filtering.

As described above, when it is determined that the target pixel is included in the block boundary or block boundary aware asymmetric adaptive loop filtering is performed by other conditions, the adaptive loop filtering unit may determine the filter shape according to the position of the target pixel. There is (S820).

For example, when a target pixel is included in a block boundary, reference pixels of adjacent blocks may be limitedly used. As a specific example, only pixels of the first column of the reference pixels of the adjacent block may be used.

As another example, columns of reference pixels of adjacent blocks may be used as far as a target pixel is located at a block boundary. As a specific example, if the target pixel is located at the second pixel position at the block boundary, up to the second column of the reference pixels of the adjacent block may be used.

As another example, the filter shape may be derived according to a method defined in the decoder, or may be determined by receiving information on the filter shape from an encoder.

When the filter shape is determined as described above, the adaptive loop filtering unit may calculate a filter coefficient according to the filter shape (S830).

The adaptive loop filtering unit may perform adaptive loop filtering using filter coefficients (S840).

Meanwhile, when the target pixel is not included in the block boundary according to step S810, the adaptive loop filtering unit may perform filtering according to an existing loop filtering method (S850).

As another example, even when it is determined that block boundary-aware asymmetric adaptive loop filtering is not performed by other conditions, the adaptive loop filtering unit may perform filtering according to an existing loop filtering scheme.

In an embodiment of the present invention, the adaptive loop filtering unit may determine whether the target pixel to be filtered is included in the block boundary. Here, the block boundary may include at least one of a picture boundary, an LCU boundary, a CU boundary, a TU boundary, a PU boundary, and the like.

In another embodiment, the adaptive loop filtering unit may determine whether to perform block boundary or asymmetric adaptive loop filtering to which the present invention is applied by checking other conditions. Here, the other condition may be a characteristic of a block or a unit, and for example, a prediction mode, a motion vector difference between blocks, a presence of a residual signal, and whether an ALF is applied.

Referring to FIG. 9, first, the adaptive loop filtering unit may determine whether a target pixel to be filtered belongs to a block boundary (S910). For example, the block boundary may include at least one of a picture boundary, an LCU boundary, a CU boundary, a TU boundary, a PU boundary, and the like.

If the target pixel to be filtered is included in the block boundary, the adaptive loop filtering unit may check a prediction mode (S920). For example, it may be determined whether the prediction modes of adjacent blocks are the same. As a specific example, it may be determined whether the prediction modes of the block F and the block G of FIG. 7 are the same.

If the prediction modes of adjacent blocks are not the same, the adaptive loop filtering unit may perform block boundary aware asymmetric adaptive loop filtering.

Alternatively, the adaptive loop filtering unit may additionally check other conditions. For example, it may be checked whether the motion vector difference between the blocks is smaller than a preset threshold (S930).

If the motion vector difference between blocks is smaller than a preset threshold, the adaptive loop filtering unit may perform block boundary aware asymmetric adaptive loop filtering.

Alternatively, the adaptive loop filtering unit may additionally check other conditions. For example, the adaptive loop filtering unit may determine whether a residual signal exists in an adjacent block (S940). For example, by checking whether the CBF (F) is 1, whether the residual signal exists in the adjacent block F may be checked.

If there is a residual signal in an adjacent block, the adaptive loop filtering unit may perform block boundary aware asymmetric adaptive loop filtering.

Alternatively, the adaptive loop filtering unit may additionally check other conditions. For example, the adaptive loop filtering unit may check whether ALF is applied (S950).

If the ALF is not applied, the adaptive loop filtering unit may perform block boundary-aware asymmetric adaptive loop filtering (S970).

On the other hand, if the target pixel to be filtered does not belong to the block boundary or other conditions are not satisfied, the adaptive loop filtering unit may determine that the target pixel to be filtered is not included in the block boundary, and accordingly, the existing loop Filtering may be performed (S960).

For example, the target pixel to be filtered does not belong to a block boundary, the prediction modes of adjacent blocks are the same, the motion vector difference between the blocks is not smaller than a preset threshold, or there is no residual signal in the adjacent block, When ALF is applied, the adaptive loop filtering unit may perform existing loop filtering.

In FIG. 9, all of the above conditions are described. However, the present invention is not limited thereto, and it may be determined whether to perform block boundary recognition asymmetric adaptive loop filtering based on at least one of the above conditions.

In addition, one of the above conditions may be used or optionally a plurality of combinations may be used, the order of the above conditions may be changed, and other conditions may be added that may include the characteristics of the block.

Referring to FIG. 10, when the target pixel (the current pixel) in the block G is included in the block boundary, the pixel of the block F may be limitedly used when the adaptive loop filter is applied. In this case, block F and block G may have different block characteristics.

For example, when a target pixel is included in a block boundary, reference pixels of adjacent blocks may be limitedly used. As a specific example, referring to FIG. 10A, only pixels of a first column of reference pixels of an adjacent block may be used.

In addition, referring to FIG. 10B, even when the target pixel is located at the second pixel position at the block boundary, only pixels of the first column among the reference pixels of the adjacent block may be used.

In the present invention, the shape of the filter may be changed as shown in FIGS. 10 (a) and 10 (b) according to the position of the target pixel. The same applies to the case where the target pixel is adjacent to the horizontal boundary.

FIG. 11 illustrates an embodiment of another filter shape referring to two column pixels among the pixels of the block F. Referring to FIG.

Referring to FIG. 11A, even when a target pixel is included in a block boundary, the present invention may use two column pixels among reference pixels of an adjacent block.

In addition, referring to FIG. 11B, the present invention may use two column pixels among reference pixels of an adjacent block as far as a target pixel is located at a block boundary.

The shape of the filter may be changed as shown in FIGS. 11A and 11B according to the position of the target pixel. However, since FIG. 11B is the same as the conventional adaptive loop filter, it may be processed in the same manner as the filtering of the pixel not including the block boundary.

However, when the filter tab is larger than the embodiment of FIG. 11, the shape of the filter may be changed. The same applies to the case where the pixel is adjacent to the horizontal boundary.

As shown in FIG. 10 and FIG. 11, the present invention may transmit information about a method of referring to pixels of an adjacent block to a decoder, and may not transmit information when an encoder / decoder uses the same method. .

12 illustrates an embodiment of a filter shape when a plurality of adjacent blocks are referenced.

Referring to FIG. 12A, the target pixel in the block G is not included in the block boundary between the block F and the block J, which are the left block, and the K block is included as the block boundary.

One embodiment of the present invention may apply filtering using all pixels of adjacent blocks (blocks F and J) when not included as block boundaries, while adjacent blocks (blocks) when included as block boundaries. K) pixels can be used on a limited basis.

Referring to FIG. 12B, only the block F, which is the left block of the target pixel in the block G, is not included as the block boundary, and the block J and the block K are included as the block boundary. As shown in FIG. 12A, in the case of blocks J and K included as block boundaries, pixels for filtering are limited.

On the other hand, in another embodiment to which the present invention is applied, the filter coefficient calculating step is a step of adjusting the filter coefficient to match the filter shape determined in the filter shape determination step.

When the filter shape is changed in the filter shape determination step, since the filter coefficient at the corresponding position is changed to 0, the total sum of all filter coefficients may not match the scaling value for filtering.

In addition, when calculating the coefficient of the adaptive loop filter in the encoder, it is calculated based on a case in which the filter shape is normally symmetric. Therefore, when the filter shape is changed, the coefficient needs to be scaled to fit the full scale value.

A filter coefficient calculation method according to an embodiment of the present invention is shown in Equation 1 below.

Equation 1

Here, S _factor means a scaling value for filtering. MASK (i, j) is a mask value indicating the presence or absence of filter coefficients. For example, MASK (i, j) has a value of 1 when there is a filter coefficient and a value of 0 when no filter coefficient exists. f (i, j) means filter coefficients having a normally symmetric filter shape, and f _n (i, j) means filter coefficients calculated by the filter coefficient calculation method proposed by the present invention. In the above embodiment, the size of the filter shape may be MxM.

Unlike the method of calculating the filter coefficients in the filter coefficient deriving step, the decoder separately separates the adaptive loop filter coefficients for the pixels including the block boundary and the adaptive loop filter coefficients for the pixels in the inner region not including the block boundary. You may have it.

As described above, the embodiments described herein may be implemented and performed on a processor, microprocessor, controller, or chip. For example, the functional units illustrated in FIGS. 1 to 3 and 11 may be implemented and performed on a computer, a processor, a microprocessor, a controller, or a chip.

In addition, the decoder and encoder to which the present invention is applied include a multimedia broadcasting transmitting and receiving device, a mobile communication terminal, a home cinema video device, a digital cinema video device, a surveillance camera, a video chat device, a real time communication device such as video communication, a mobile streaming device, Storage media, camcorders, video on demand (VoD) service providing devices, internet streaming service providing devices, three-dimensional (3D) video devices, video telephony video devices, and medical video devices Can be used for

In addition, the processing method to which the present invention is applied can be produced in the form of a program executed by a computer, and can be stored in a computer-readable recording medium. Multimedia data having a data structure according to the present invention can also be stored in a computer-readable recording medium. The computer readable recording medium includes all kinds of storage devices for storing computer readable data. The computer-readable recording medium may include, for example, a Blu-ray disc (BD), a universal serial bus (USB), a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, and an optical data storage device. Can be. The computer-readable recording medium also includes media embodied in the form of a carrier wave (eg, transmission over the Internet). In addition, the bit stream generated by the encoding method may be stored in a computer-readable recording medium or transmitted through a wired or wireless communication network.

As mentioned above, preferred embodiments of the present invention are disclosed for purposes of illustration, and those skilled in the art can improve and change various other embodiments within the spirit and technical scope of the present invention disclosed in the appended claims below. , Replacement or addition would be possible.

Claims

In the method for adaptively filtering according to a block boundary,

Determining whether a current pixel is included in a block boundary;

If the current pixel is included in a block boundary, determining a filter shape according to the position of the current pixel;

Calculating filter coefficients based on the filter shape; And

Performing adaptive loop filtering using the filter coefficients

Method comprising a.
The method of claim 1, wherein

Determining whether at least one other condition relating to the block characteristics is satisfied

More,

And if the at least one other condition is met, the adaptive loop filtering is performed.
The method of claim 2,

At least one other condition associated with the block characteristic indicates whether the interblock prediction mode is different.
The method of claim 2,

The at least one other condition associated with the block characteristic indicates whether the difference between the block motion vector is less than a predetermined threshold.
The method of claim 2,

At least one other condition associated with the block characteristic indicates whether a residual signal exists in an adjacent block.
The method of claim 2,

At least one other condition associated with the block characteristic indicates whether adaptive loop filtering is not applied.
The method of claim 1,

Wherein the filter shape is determined by reference pixels that are used on a limited basis, and wherein the reference pixels change according to the position of the current pixel.
The method of claim 7, wherein

The limitedly used reference pixels are determined in units of columns adjacent to a block boundary.
The method of claim 7, wherein

And wherein the limitedly used reference pixels are determined in proportion to the position of the current pixel away from the block boundary.
The method of claim 1,

The filter shape may be derived according to a method defined in a decoder or determined by receiving information on the filter shape from an encoder.
The method of claim 1,

The filter coefficients are calculated using the scaling coefficients corresponding to the filter shape.
An apparatus for adaptively filtering according to a block boundary,

A block boundary determination unit determining whether a current pixel is included in a block boundary;

A filter shape determiner configured to determine a filter shape according to a position of the current pixel when the current pixel is included in a block boundary;

A filter coefficient calculator for calculating filter coefficients based on the filter shape; And

Filtering unit for performing the adaptive loop filtering using the filter coefficients

Apparatus comprising a.
The method of claim 12,

The block boundary determiner determines whether at least one other condition related to a block characteristic is satisfied,

And if the at least one other condition is met, the adaptive loop filtering is performed.
The method of claim 13,

At least one other condition associated with the block characteristic is whether the interblock prediction mode is different, whether the interblock motion vector difference is less than a predetermined threshold, whether there is a residual signal in an adjacent block, or an adaptive loop At least one of whether filtering is not applied.
The method of claim 12,

Wherein the filter shape is determined by reference pixels that are used on a limited basis, and wherein the reference pixels change according to the position of the current pixel.